1. Field of the Invention
The present invention relates to a physical vapor deposition (PVD) device, and more particularly, to a PVD device for forming a uniform metal layer on a semiconductor wafer.
2. Description of the Prior Art
The physical vapor deposition process is employed to form a thin metal layer on the semiconductor wafer in the semiconductor fabrication process. The metal layer with no void formed inside and with good uniformity in its thickness is considered as a metal layer with good structure.
Please refer to FIG. 1, FIG. 1 is a schematic diagram of a PVD device 10 according to the prior art. The prior art PVD device 10 comprises a chamber 12 in which air is evacuated and then argon (Ar) gas is introduced for generating Ar ions 18, a wafer chuck 14 for holding a circular-shaped semiconductor wafer 16, and a circular-shaped metal target 17. The PVD device 10 generates a built-in electric field inside the chamber 12, causing the Ar ions 18 bombard the metal target 17 to release metal atoms 19 from the metal target 17. Then, the metal atoms 19 drop onto a semiconductor wafer 16 below the metal target 17 so as to form a metal layer 20 on the semiconductor wafer 16.
Please refer to FIG. 2, FIG. 2 is a schematic diagram of the formation of the metal layer 20 on the semiconductor wafer 16 by using the PVD device 10 shown in FIG. 1. The circles represent the metal atoms 19 released from the metal target 16 caused by ion bombardment, and the arrows represent the directions of the drop of the metal atoms 19 in FIG. 2. During the PVD process for the semiconductor wafer 16 by using the PVD device 10, the directions of the drop of the metal atoms 19 are random. The semiconductor wafer 16 comprises a gap 22, and a gap 24 with a higher aspect ratio. Thus, an overhang issue occurs in the metal layer formed on the top corner of the gap 22, and a void 26 is formed within the metal layer inside the gap 24. Therefore, the metal atoms 19 generated inside the PVD device 10 have worse gap-filling capability for forming the metal layer.
Please refer to FIG. 3 and FIG. 4, FIG. 3 is a schematic diagram of a PVD device 30 with a collimator according to the prior art. FIG. 4 is a schematic diagram of the formation of a metal layer 40 on a semiconductor wafer 36 by using the PVD device 30 shown in FIG. 3. The difference between the PVD device 30 and the PVD device 10 is that the PVD device 30 further comprises a collimator 38 having many parallel pipes. The collimator 38 obstructs the metal atoms released from the circular-shaped metal target 37 and causes the metal atoms to drop in vertical directions. When the PVD device 30 is employed to process PVD on the semiconductor wafer 36 comprising gaps, most of the random metal atoms are obstructed by the collimator 38, but a few metal atoms 19 drop onto the semiconductor wafer 36 in the vertical direction. Therefore, less overhang issue occurs in the metal layer 40 formed on the semiconductor wafer 36. That is the metal atoms 19 generated in the PVD device 30 have better gap-filling capability for forming the metal layer. However, since most of the random metal atoms are obstructed and remained in the collimator 38 , the collimator 38 need to be scoured or replaced regularly, resulting in the increases of time and costs of the PVD process.
Please refer to FIG. 5, FIG. 5 is a schematic diagram of an ionized metal plasma (IMP) PVD device 50 according to the prior art. The difference between the IMP PVD device 50 and the PVD device 10, 30 is that the IMP PVD device 50 comprises an annular metal coil 58 positioned between the metal target 57 and the wafer chuck 54. During a PVD processing by using the IMP PVD device 50, an AC (alternating current) voltage of radio frequency is supplied to the metal coil 58 to generate a magnetic field in the cylindrical region surrounded by the metal coil 58. The built-in electric field of the IMP PVD device 50 causes the Ar ions to bombard the circular-shaped metal target 57 to release the metal atoms from the circular-shaped metal target 57 inside the chamber 52. Meanwhile, the magnetic field causes the Ar ions to move in a spiral direction to increase collisions between the Ar ions and the released metal atoms, resulting in the reduction of the mean free path of the Ar ions and the ionization of the metal atoms. Then, the vertical built-in electric field in the PVD device 50 causes the ionized metal atoms to increase their dropping velocity of in the vertical direction, and relatively decrease their dropping velocity in other directions. Therefore, the metal atoms created in the IMP PVD device 50 have better gap-filling capability during sputtering the semiconductor wafer.
Please refer to FIG. 6, FIG. 6 is a schematic diagram to show the relative thickness along line a--a of the metal layer formed on the semiconductor wafer in the IMP PVD device shown in FIG. 5. Because the size of the chamber 52 for the IMP PVD device 50 is fixed, which limits the size of the annular metal coil 58, so that the thickness of the metal layer formed on the peripheral portion of the semiconductor wafer 56 is smaller than that of the metal layer formed on the center portion of the semiconductor wafer 56. Consequently, the metal layer formed on the semiconductor wafer 56 by using the IMP PVD device 50 has poor uniformity for its thickness.